Styrene is an important monomer used in the manufacture of many plastics. Styrene is commonly produced by making ethylbenzene, which is then dehydrogenated to produce styrene. Ethylbenzene is typically formed by one or more aromatic conversion processes involving the alkylation of benzene.
Aromatic conversion processes, which are typically carried out utilizing a molecular sieve type catalyst, are well known in the chemical processing industry. Such aromatic conversion processes include the alkylation of aromatic compounds such as benzene with ethylene to produce alkyl aromatics such as ethylbenzene. Typically an alkylation reactor, which can produce a mixture of monoalkyl and polyalkyl benzenes, will be coupled with a transalkylation reactor for the conversion of polyalkyl benzenes to monoalkyl benzenes. The transalkylation process is operated under conditions to cause disproportionation of the polyalkylated aromatic fraction, which can produce a product having an enhanced ethylbenzene content and reduced polyalkylated content. When both alkylation and transalkylation processes are used, two separate reactors, each with its own catalyst, can be employed for each of the processes.
Ethylene is obtained predominantly from the thermal cracking of hydrocarbons, such as ethane, propane, butane, or naphtha. Ethylene can also be produced and recovered from various refinery processes. Thermal cracking and separation technologies for the production of relatively pure ethylene can account for a significant portion of the total ethylbenzene production costs.
Benzene can be obtained from the hydrodealkylation of toluene that involves heating a mixture of toluene with excess hydrogen to elevated temperatures (for example 500° C. to 600° C.) in the presence of a catalyst. Under these conditions, toluene can undergo dealkylation according to the chemical equation: C6H5CH3+H2→C6H6+CH4. This reaction requires energy input and as can be seen from the above equation, produces methane as a byproduct, which is typically separated and may used as heating fuel for the process.
In view of the above, it would be desirable to have a process of producing styrene that does not rely on thermal crackers and expensive separation technologies as a source of ethylene. It would further be desirable to avoid the process of converting toluene to benzene with its inherent expense and loss of a carbon atom to form methane. It would be desirable to produce styrene without the use of benzene and ethylene as feedstreams.
Alternatively, toluene has been used to produce styrene with either methanol or methane/oxygen as the co-feed. Theoretically methanol (CH3OH) and toluene (C6H5CH3) can be reacted together to form styrene, water and hydrogen gas, as shown below:CH3OH+C6H5CH3→C8H8+H2O+H2 
In practice, however, the methanol (CH3OH) often dehydrogenates into formaldehyde (CH2O) and hydrogen gas (H2). Often the toluene conversion is low or the methanol reacts too rapidly and the selectivity on the methanol is too low to make the process economical. In order to avoid this undesirable side reaction, a method of producing styrene without the use of methanol would be desirable.